PLAASTIC’ITY OF D E S T X L I M P K E S S I O S C O l I P O U S D BY TV.iLTER 5 . C R O W E L L A S D ALBERT S . i U Y D E R S J R .
*
**
Ijental impresfion compound consists of a wax-like inixture of stearic acid, resins, and oils intimately mixed Tvith an inert filler, like talc and a pigment. The unfilled organic part is called conipound u’ax. In use, the compound is softened in hot water, placed in a tray and forced against the interior surface of the mouth. Tht. tray is allowed t o remain in the mouth until its contents harden. It is withdrawn and the impression poured with plaster of paris. After the plaster sets. the compound is again softened in hot xater and stripped from the iiiodel. The object of a dental impression is obviously to secure a niodel upon which an appliance can be built which will give continuously satisfactory service in masticating the food. The appliance must be constructed so that it does not interfere with the normal functioning of surrounding organs, and is not displaced h y their motion. It must conform closely t o the tissues against which it is placed. not only when the tissue is relaxed or at rest, but inuch more importantly, when the tissue is compressed by the appliance under the stress of mastication. Plaster of paris is the original impression material. There is no question that it will give an accurate reproduction of the surfaces against which it is placed. Plaster is soft and fluid. I n practice. the impression obtained by its use is a copy of the tissues in a relaxed condition. This gives a model frequently differing at certain points form the aspect of the tissues compresqed under the plate by the act of mastication. After it has set, a plaster impression can be altered only by scraping or cutting. The outer rim of the impression copies the muscular attachments between the cheeks and the alveolar ridge in one position. -Attempts to conform the rim to the motion of the inuscles in mastication by carving are seldom sufficiently accurate t o be satisfactory. The need for ensuring freedom of motion of these muscular attachments, when the plate is in place, led to the developnient of modelling compound impression technique. TT-hen compound is used, the rim of the impression can be softened by heat, the impression returned to the mouth and, by moving the muscles, clearance can be cut by the muscles themselves. This “muscle trimming” of the rim of the plate is one of the main features of the compound impression technique developed by the Green Brothers a number of years ago. The importance of tissue compression was not recognized at once. While cornpound impression techniques gave better fitting, more serviceable plates, the improvement was attributed largely t o the lack of interference of the plate with the surrounding organs, particularly the muscles referred ~~
~~
* Paper presented a t the Plasticity Symposium, Lafayette Collegr, Oct. 17 (1924).
* * Contribution from the Research Latmratory
pany.
of t h e 5. S. \I7hite Dental Mfg. Corn-
1268
\V.lLTLR
h.
CROTTELL .4XD ALBERT S A r S D E R S , J R .
to above. Any interference between the plate and these muscles woultl of course, tend t o displace the plate, break the "suction" and cause the plate t o drop. I n the Green technique, considerable compression was incidentally ohtained because of the necessity for firmly forcing the compound honie. ('ompression plays a large part in the recent V7adsworth technique in which the tissue is pressed into the impression by the actual power of the masticating muscles. It is apparent that a dental impression compound mu,it d o ivcrc than simply copy the surface against which it is placed. Its plastic properties must be carefully regulated. *At the working temperature, it must be mobile enough t o allox of muscle trimming, yet have a yield value high enough t o ensure adequate and proper tissue compression, At some temperature above that of the mouth it must he essentially rigid, so that it can be withdrawn from the mouth without distortion. It has long been known that temperatures up to 50°C'. can be readily tolerated by the mouth, while temperatures above j jo('.cause discoinfort. Therefore the working range of an impression compound is between the upper limit of toleration, js0C and the temperature of the mouth, 37. j0C. The plastic properties of an impression conipound are a function of the plastic or fluid properties of the compound was and the quantity of talc added. As noted above. the compound wax is the mixture of stearic acid, resins and oils combined by fusion. \\'hen this mixture is heated it becomes fluid. On caoling a temperature is reached a t which stearic acid commences t o crystallize from the melt. The separation of stearic acid in itself reduces the fluidity of the melt acting like any other suspended solid. Furthermore, the fluidity of the remaining mother liquor is rapidly reduced by withdrawing from it one of its most fluid constituents. The net result is a rapid hardening or setting of the was in a narrow temperature range. Broadly, the temperature at which crystallization begins depends upon the quantity of stearic acid present. I n practice, supercooling is 1)ouncl to occur depending upon the fluidity of the particular compound studied at its crystallization point. The fluidity of the compound also affects the rate a t which the stearic acid separates. These two factors determine the setting time of the wax and hence the compound. The temperature of crystallization must be above mouth temperature, otherwise the compound will not develop sufficient rigidity t o be withdrawn from the mouth without distortion. The addition of talc or other inert filler t o the was rapidly reduces the fluidity of the misture and, above a certain volume, concentration changes the mixture from a liquid to a plastic material with a finite yield value. The complexity of the problem and the lack of data in the literature on similar substances necessitated en elaborate experimental study in order to secure reliable quantitative data. ~
PLASTICITY O F D E S T A L I M P R E b S I O S C’OXPOUSD
1260
Both waxes and compounds are studied in the teiiipemtnrc ranpc of 103 ( ’ to 56°C’. Preliniinar?- studies indicated that wax S o . 4 antl t h e conipouncis niadc from it were reasonably satisfactory. It, gave favorahlr results on both l a b oratory and field tests. This material was selected for detailed study. The compound was contained 3 :c:p stearic acid, the reiiiainder being resin. Its crystallization point (lip undisturbed cooling) was AT.^'('. I t was iiiisetl Kith talc and pigment in the following proportions: C‘cmpounti So.
Tulr. IVeipht I’crcrnt,
4( *
4
30.4 31.3
11,3 28.5
-L\
33.2
31.0
‘l’:~lc,.
__
I’t~ri~ont.
The density of the talc-pignient mixture used n-as 2.6\; On the was wc set out t o determine
I-Fluidity-triiiperature relation between ,ij 3 ant1 36”. 2-Temperatur~ of zero fluidity. The floTv-shear relations of the three conpounds n w e stiitlictl in the saiiie tciiiperatmurerange and the results used t o tleteriiiirie 1-Effect of t,alc on the fluidity of mixture. 2--Volunie concentration of talc giving “zcro” fluidity. ,i--Effect of talc on yield value. 4-Fluitiity. mohility and yield valuc. tciiiprratwc rclstion of tlir three conipoiind,~between 3 7’ and 56’. l h e plastic properties of -the wax antl the coiiipountls were stutlietl liy the capillary tube method described by Biriphani. “Fliiitlitj* arid Plasticity”, p. 3 IO. The dimensions of the capillaries arc given in Table I. Lengths w r e iiiensiireti. liy a niicronietcr anti. radii under a niicroscopc q u i p p r d with a fila8r niicrometer.
TABLEI 1)iiiiensions of Capillaries Usetl Capillat~ySo
Radius Cm.
I~crlgrllC’lll.
11
0.1581
7.635 i 584 T .:49
17
O.Oj.57
7.922
2 1
0.0808
T
7
0.0782
I ,i
0.Ijj2
,
’
983
Apparatus The apparatus used in o ~ i rriieasuremeiits was essentially that described by Ringham in “Flutliity anti Plasticity”, p. 305.
I2j 0
IV.kLT€.R
h.
CROITXLL A S D ALBERT S A T S D E R S , J R .
Method of Measurement Keighecl glass test tiibec were attached t o the pla~tonietersby ineanr of rubber stoppers. The plsstonieter containing the coiiipourid was then placrtl in the bath, previously set at the desired temperature, and allowed to remain for one hour at that temperature. The temperature of the bath was determined by ineans of a thermometer calibrated at 3 2 . 4 Y ‘ . the transition temperature of S a z S O l I O HzO. After standing one hour, air from the stabilizing reservoir was admitted above the coinpound through a needle valve. A stop watch, reading t o I ;. second was started. -15 soon as the mercury in the manometer became stationary, the height in m s . of mercury was noted. Thr pressure was maintained constant during a tieteriiiinat ion by hand regulation. df‘ter a sufficient time, usually 50 nimutrs. had elapsed, the manonieter was again read, the pressure shut off. the watch stopped, and the plastometer turned t o air by ineans of another needle valve. I h i n g the elapsed time the temperature of the air surrounding the iiianoiiieter was noted from time t o tiiiie. The plastoineter was taken out of the hath and the glass tube containing the extruded compound removed and placed in an electric oven at 5o0C‘, for one hour in order t o reinove moisture. dfter cooling, the tuhrl was again weighed and the weight of the extruded qample noted. Calculations Fluidities ivere calculatetl from the following formula: p Q
IT’ I
= = =
p = p =
R
=
t =
=
81K 7rgR4pt P ~
Fluidity Keight of extruded conipountl Length of capillary Density of compound Pressure in gins. ’ciii. Radius of capillary Time
(’orrectionq were applied for hydrostatic head and room teniperatur(x. Corrections in pressures for various temperatures and niercurical heights w e r ~ taken from Ringham’s “Fluidity and plasticity”, p. 302. 81K Plasticities were calculated by the forniula p = TqR4ptiP-p) p the “yield pressure” was cleteriiiinecl by calculating the intercept on the pressure avis of the weight per second ~ s pressure . curves by the method of least squares. Yield values were calculated froin yield prebsures by the rxlntion f = Pli 2 L Notes on Measurements The measurement of the fluidity of the Tvaxes above 46°C‘.presented no particular difficulty. Sonie trouble was experienced in obtaining inaterial free from suspended impurities but this was finally overcome by straining
PL.1bTICITT O F D E S T A L 13IPREbSIOS C O M P O U S D
1271
the molten wax through a zoo niesh screen. The .oj and .08 cni. capi1l:arieq were convenient for this work arid gave corresponding results. Below 46°C concordant results w r e not obtained The fluidities iiicasw e d were very low and tlic crrors of' iiieasurenient sufficieni t o niai~kc~lly affect the results. \J-e n-ere unahle t o accurately determinc the tempcraturc of zero fluidity either directly or kiy exterpolation of the tcniperatiirr, fluitlity curves. The deterinination of yield value and mobility of tlie conipountl~was still ~iu.~re difficult and uncertain. The niohilities are about I I O the niagnitutlr of the fluidities ineasuretl. The .os cni. capillaries could not lie u.etl. Better results were obtained with the .08 cin. capillaries 1)ut they n-ei't~-till \-cry divortlant . l l i i c h of the irregularity was attributed to the presence of relativelj- large ~uspeiidetlparticles. Their effect should be minimized tiy using larger ca1)illarieg. Accordingly soiiie . I j cni. tubes were secured. The results olitaincd with these tubes were much better, but not in agreement with those obtained n-ith the smaller tubes. the yield value tieing much higher. \\'hen t h k was called t o the attention of Bingham (private coinmunication) he pointed out that with the large tubes. the shearing forces mere niiich greater thari thosc ~iiiployedwith the small tuhes. The flow-shear curve at low pressure tlcparts from liniarity and approaches the pressure axis asymptotically. This was confirmed hy observations with .08 cin. capillaries at high pressures (and correspondingly high shearing forces.) Under these conditions. the result. wcre in agreement with those obtained with large capillaries. This departure of the floiv-shear ciirve from linearity deserves specid cwmnient. Wiile theoretically two iiieasiirements arc required t o determilie yicltl value and mobility, in practice at least three nieasurrnients should tie niacle :ti widel>- differing shearing stresses so that any departure from linearity will be observed aiid the yield ~ a l u edetermined froin nirasureiiients at shears high enough t o be in the apparently linear region of thc ciirvc. - i t liest. the resiilts ohtainetl were iiot in good agreenieiit. A-lpparently in our hantls. tlie apparatus is incapable of measuring the wia11 mobi1itic.i ivt' met t o an accuracy of over I.
F~;.
Sources of Error of the sample. This was controlled 11) prolongetl 111c-
r-Iiilioiiiogeneity c1i:tnical mi\ing. a-Suspencletl foreign particles. Controlled by screrning h t h wax+ alitl coinpounds through appropriate sieves, under pressure. trapped air. Screening tends to break up air buhhles antl reduce- t h i i error t o a large extriit. I n filling the plastonieter, however. air is likel). to br cntrained. 11ost but riot all of it is eliiiiinated by continued heating of t h r plastometer before running the first determination. 4-Errors in weighing the extruded inaterial. Considerablc error iiiay i)r intorducetl 11)- not efficiently cleaning and drying the receiver antl by not
I272
WALTER h. CROWELL A S D ALBERT SAUSDERb, J R .
conipletely drying the extruded material t o constant weight. The quantities weighed were seldom over I z gm., hence it was necessary t o take precautions in cleaning and drying, which mould be quite unnecessary with larger wights. i-Flow of compound under its own head. -After the pressiw has txwi shut off there is a slight t,endency for the coinpound to continue to flon- in minute quantities. This error is very sinal1 arid may he reducetl by removing the plastorneter from the bat'h anti chilling the receivcr in icc n-ater as soon as t'he pressure is shut, off. 6-Removing t'he receiver before the conipoiind has solidified. .Is thc compound has a tendency to adhere t o the walls of the wighing hottlr. when soft, a sniall port'ion of the compouiicl may he pulled out of the capillary if it, is not allowed to first become solid. i-('lltti11g the compound from the end of the capillary. The coiiipouncl is extruded from the capillary in the form of a thread. IYhen the compound has solidified and the receiver is removed, a portion of this thread is attached t o the compound in the capillary. Unless care is exercised in cutting the coinpound from the capillary, a small portion of the conipoiuid n-ill be broken off within the capillary giving a high result. 8--Fluctuation of bath temperature. This is a serious suurce of error, difficult to control when the experiment requires jo minutes or niorc. .Autoniatic regulation with toluol regulators proved unsatisfactory antl addit'ional hand regulation was resorted to. A part ~f thc variations in our resiilt's may he attributed t o this cause. ()-Leakage in the systein. This tion of all joints.
guarded against hy frcqucnt iiispw-
Io-Errors in the hytlrostafic head correction and the nianonieter reading. d t the high pressures used these errors are negligahle. Many of these sources of error would he negligable i l l the i i i sureinent ~ of high motilities at low pressures but untler our cxperimcntal conditions they are serious causes of deviation.
The changes in the structure of the was noted above are reflected in t h ( > temperaturc fluidity relations. &It high teniperatiires thc wax is a liquid. As the temperature falls the fluidity decreases, but not in a linear fashion. Thew is some sap t o the curve. This can be a t t r i h t e d t o interiial changcs in thp liquid similar t o those in water antl other associated liquids which show a siniilar sag t o the temperature fluidity curves. about 46.5' the fluidity coiiiniences t o rapidly decrease, finally reaching extremely lo^ v a l u ~ s . This changr. occurs in a range of two or threr degrees antl corresponds to the separation of the stearic acid. The nielting point of t,he was cleterniined by unclisturbetl cooling is 11.q0, while the teniperature at which stearic acid begins t o separate as indicated by fluitlit\- measurements is 46. jo. The value determined hy iintlisturlietl
PLA\TICITY O F D E S T A L I J I P R E s a I O S C O J I P O U S D
TABLEI1 Fluidity of Compouiid JT-as S o . 3Ianometci, rending in mm. hg.
Dcn.1 t \
113.9' 113,yS
0
9900
0
115.69
0
119. I O
0
I49 ' 03 I j o . 15
0
9900 9900 9900 9900 9900 9900 9900 9900 9900 9900 9900
2 10 3 .i 207,63 '
2 5 7 . I8
2 5 8 . I,; 298.00 3 0 0 . ,;8
0 0 0
0 0 0
108.62
9880 o 9880
108.21
o 9880
141.08
o 9880
1 4 4 .O
i
o 9880
148.37 201.3'
o 9880
IO1
.go
0
o
o 9880
200.6.;
o 9880
259.00
o 9880
259.30 296.72
o 9880
o 9880
118.86
0
987,
21.5.9.5
0
304.T 1
0
9877 9877
95.60 206. 2 7
0
199.i o
0
303.93
0
0
9843 9843 9843 9843
4
1274
\\-..1LTER
S. CROlYELL A S D .SLBERT 8.4UKDERS, J R ,
TABLE11 (continued) r .
1e m p
48.0 48.0
48.0 48.0 48.0 48.0 48.0 48.0 48.0
JIanoxncter rending in mm. hg.
10g,oo 110.62 I64,93 168.34 213.38 214.49 263.06 300.60 302,84
D(,npity
0.9790 0,9790 0.9790 0.9790 0,9790 0.9i90 0.9790 0.9790 0.9790
\Vt. pw sw.
I G ~Fluidit,yX 10"
68.71 72.33 109.o.j I I O , 41 139.26 138. 5 5 168.66 179.44 185.95
Mean 50.0
102.06
50.0
50.0
104.93 106.42 167.87
.jO. 0
208.23
0.9735 0,9735 0,9735 0.973.; 0 ,9 73 5
50.0
213.68 252.39 271.40 303 . i 1 304.16 305.68 306.41
0.9735 0 . 9i 3 5 0.9735 0.9735 0.9735 0.9735 0,9735
50.0
50.0
jo.0 50.0 50.0 50.0
50.0
06 10.45 10.61 IO. j r 10.49 10.38 10.31 9.60 9.87 IO.
Q
5 2 . 0
117.01
52.0
220.53
52.0
221.28
52.0
226.47
0.9680 0.9680 0.9680 0.9680 0.9680 0.9680 0.9680 0.9680
5 4 . ooc j4.0 54.0
116.68 119.91 119. 5.5 211.88 215.76
0.962.; 0.962; 0.9625 0.9625 0.9625
52 52
.o .o
52.0
54.0 54.0
108.OS 111.34 1 1 2 . I1
113.64
10.2
88.71 92 2 7
93 06 13T 94 169 00 178 30 208 84
53 248 69 251 97 223
2 5 5 01
259 49
Mean 9 j2.0
=
118 91 125.83
130.80 126.72
=
17.79 I8,29 18.8; 17.63
134.06
18.49 17.91 18.87 2 5 6 97 17.67 246.09 Mean cp = 18.I 9 243 03
I
j6.16
161.22 165.63 280. 2.3 284,;r
Mean
21.83 21.93; 22.55
21.67 5;
21.
p = 21.91
PL. I P R E b S I O S CO>IPOUSI)
I281
Compound No.4 A
-1few tieteriniriations have been made at a still higher talc content. The yield value has increased and the mobility fallen off. These results arc not sufficiently reliable t o certainly indicate that the yield value is a linear function of the volume percent of talc. It is in our opinion, however. that qllch is the caqe. (see Fig. 4).
Conclusions The fluidity-temperature relations of compound wax reflect the physical changes n-hich occur. The crystallization of the stearic acid is indicated 13y a sharp break in the curve. The temperature of crystallization can be deterniined to within about 1°C. by fluidity measurements. The addition of talc
rapidly reduces the fluidity of the mixture. Above 19 89 volume percent talc, the mixture develops plastic properties. At 28.; volume percent talc, the yield value is 2.69 gm cm'. 1,arg-t. additions of talc increase t,he yield value in a s~eniingly lirwar manner. Our nieasurements do not show that yield value depends upon teniperature. It seems to be characteristic of the quantity of filler added alone. The forni of the fluidity-temperature curve of the wax is riot changed hv the addition of talc but the values of the fluidity are rapidly reduced. When n-orking with plastic substances it is necessary t o inake ineasiirenients at several widely separated shearing forces so that the yield valuc can be deterniined from rneasurenients unaffected by the departure of the flow shear curve from linearity a t low pressures. K e wish t o acknowledge the helpful criticism anti advice received from Dr. E. C . Bingharn during the progress of this investigation and the support of the S. S. TT-hite Dental blanufacturinp C'ompany in carrying it out. S e p t e m b e r 18. 1,925
